Humidity plays a very major role in various industrial and commercial applications. Monitoring and controlling humidity is of great importance for the reliable operation of various systems. For example, solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors are fabricated using semiconductor processes. Humidity sensors represent but one class of semiconductor-based sensors finding a useful industrial application. Modern manufacturing processes, for example, generally require measurement of moisture contents corresponding to dew points between −40° C. and 180° C., or a relative humidity between 1% and 100%. There is also a need for a durable, compact, efficient moisture detector that can be used effectively in these processes to measure very small moisture content in gaseous atmospheres.
Humidity can be measured by a number of techniques. In a semiconductor-based system, for example, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Two of the most common physical changes are variations in resistance and the change in dielectric constant, which can be respectively translated into a resistance change and a capacitance change. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result includes erroneous readings, among other problems.
Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. It is important in the construction of capacitive elements, however, to avoid problems that can arise with certain constructions for such elements. In addition, there can also be inaccuracy incurred at high relative humidity values where high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element. By making the component parts of the element thin, it has been found that the above-mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables.
A conventional capacitive humidity sensor, in general, can include a semiconductor substrate, and a pair of electrodes, which are formed on a surface of the semiconductor substrate and face each other across a particular distance. A humidity-sensitive film may also be placed between the electrodes and formed on a surface of the semiconductor substrate. The capacitance of the film changes in response to humidity. The sensor detects humidity by detecting changes in capacitance between the pair of electrodes in response to variations in the surrounding humidity. The capacitance of the film changes in response to humidity, and the sensor detects humidity by detecting changes in capacitance between the electrodes with respect to changes in the surrounding humidity.
Humidity sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure, along with a layer or coating of a dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and attaining equilibrium in a short period of time. Capacitive humidity sensors are typically made by depositing several layers of material on a substrate material.
Referring to FIG. 1, a perspective view of the basic components of a prior art semiconductor-based humidity sensor 100 are illustrated. A semiconductor humidity sensor 100 is generally fabricated on a silicon substrate 110. The active sensor components include respective lower and upper electrically conductive plates 120, 140 sandwiching a humidity sensing medium 130, such as a polymer. The polymer material is sensitive to humidity, and its electrically conductive properties (e.g., resistance and/or capacitance) change as it absorbs moisture, or as it dries. The lower and upper plates 120, 140 can be electrically connected to sensor circuitry (i.e., not shown in FIG. 1). A protective layer 150 can be used to protect the active components of the sensor (e.g., top plate 140 and sensing medium 130) from debris 160. Upper plate 140 can be designed to be porous in order to enable humidity to enter into the sensing medium from an external environment of interest 170 (i.e., the monitored environment of interest).
Referring to FIG. 2, a cut-away side view of a prior art relative humidity sensor 200 is illustrated. The example humidity sensor 200 depicted in FIG. 2 includes a substrate 210. Insulating materials 220 can function as a buffer between the substrate 210 and respective first and second lower capacitor plates 240, 245. First lower capacitor plate 240 is electrically connected to a first connector 230. Second lower capacitor plate 245 is electrically connected to a second connector 235. A sensing medium 260 is generally disposed on top of the first and second lower capacitor plates 240, 245. A porous platinum top capacitor plate 250 is then disposed on top of the sensing medium 260. A protective layer 255 can also be disposed above the top plate 250 for protection of the top plate 250 and sensing medium layer 260. Two capacitors Cx1 and Cx2 are schematically illustrated in respective positions within the sensing medium 260 between the first lower capacitor plate 240 and top capacitor plate 250 and the second lower capacitor plate 245 and the top capacitor plate 250. The gap/barrier 265 can be between the first and second lower contact plates 240, 245, to create the series capacitor configuration for Cx1 and Cx2.
As depicted in the prior art illustration of FIG. 2, capacitor Cx1 can include a common top plate 250 (common to both Cx1 and Cx2) and a first lower capacitor plate 240 in further electrical contact with a first electrical contact 270. Capacitor Cx2 generally includes common top plate 250 as its first contact and a second lower capacitor plate 245, which is in further electrical contact with second electrical contact 275. Also illustrated is a parasitic capacitor Cct which can be located between the upper plate 250 and the silicon substrate 210. The total capacitance between the pair of electrodes in response to changes in the surrounding humidity can be expressed in the form of equation (1) as follows:CTotal=(CX1*CX2)/(CX1+CX2+Cct)  (1)
Condensation occurs whenever the surface temperature of the sensor's active area drops below the ambient dew point of the surrounding gas. The condensation of water can be formed on the sensor or any surface even if the surface temperature momentarily drops below the ambient dew point. Small temperature fluctuations near the sensor can unknowingly cause condensation to form when operating at humidity levels above 95%. Because of this, a sensor's recovery period from either condensation or wetting is much longer than its normal time response.
The problem associated with prior art capacitive humidity sensors such as, for example, sensor 200, is that the condensation of liquid water on the sensor creates a capacitive path 225 having parasitic capacitance CW with respect to the substrate 210 as depicted in FIG. 2. This effect causes the total capacitance value CTotal to decrease, and therefore generates an erroneous low humidity value. In the event of water condensation, the total capacitance value can be expressed as indicated in equation (2) below:CTotal=(CX1*CX2)/(CX1+CX2+Cct+CW)  (2)
As shown in equation (2), the total capacitance value CTotal decreases due to the presence of water condensation, which results in an inaccurate measurement of humidity.
Various packaging techniques exist which are designed to prevent condensation on the sensing surface. These can be useful, but limitations in cost and packaging do not always allow for these solutions. Hence, another solution is needed that avoids the deleterious effects of condensation without modifications to the packaging. The current invention accomplishes this by embedding the solution in the circuit without adding cost. This is accomplished by modifying the circuit so that the parasitic capacitance created by water condensation is redirected to a different part of the circuit.
Based on the foregoing it is believed that a need exists for an improved relative humidity sensor that redirects the path of parasitic capacitance due to condensation to provide a more accurate measurement of humidity.